1. Field of the Invention
The present invention relates to an optical phase grating filter for providing a cut-off of a subjective spacial frequency without diminishing the light level in the transmitted frequencies. More particularly, the present invention provides an improved two dimensional phase grating that can be utilized as an optical low pass filter for unidirectional scanning, or a low pass filter for a two dimensional target area.
2. Description of the Prior Art
Optical low pass filters have been known in the prior art, particularly in the field of color television camera systems. In color television, the transmission of a color picture representative of the object scene, requires three independent video signals. Generally, a dichroic filter is utilized to modulate the light flux for converting an object scene into high frequency signals.
Various forms of dichroic, or color encoding filters, have been utilized in the prior art, for example, U.S. Pat. No. 3,771,857, and U.S. Pat. No. 3,860,955. These filters are placed in the light path of a pickup tube, or image tube, to separate the light passing through them into primary color component light signals. These light signals are then transformed into electric signals after they have impinged upon a photosensitive element of the pickup tube. The image plane of the optical system is focused on the pickup tube and an electron beam scans a raster in deriving the electrical signals.
In addition to the primary color signals generated, the color encoding filters also provide, throughout their grid, areas that are transparent to the primary colors and thus, pass a luminance signal representative of the brightness of the image. The color signal components and luminance signals can then be electrically separated by circuitry external to the pickup, or image tube. A problem that is recognized in the prior art is the interference, or cross talk, between high frequency luminance signals and the chrominance signals. If the object scene contains high spacial frequency components which fall into the chrominance signal band, spurious signals are produced by the interference between the luminance and chrominance signals. The decoding scheme in the video system can erroneously interpret these spurious high spacial frequencies as color information, and accordingly, incorrect colors will be observed in the reconstruction of the object scene. Thus, it has been known in the prior art that it is highly desirable to eliminate any beat frequencies, and attempts have been made to optically defocus the optical image formed at the target electrode of the image tube.
One approach to solve this problem, has been to utilize an optical low pass filter to introduce phase retardation, and therefore, defocus or cause blurring. The U.S. Pat. No. 3,768,888 and U.S. Pat. No. 3,910,683 disclose the use of rectangular wave phase gratings which include a plurality of sets of laminae to attenuate striped diffraction patterns of Fresnel order of a defocussed image and color striped pattern.
An optical low pass filter utilizing phase grating to attain a response to zero in a frequency over a desired cut-off frequency, while at the same time being independent of the F number of the optical system, is disclosed in the U.S. Pat. No. 3,821,795 and U.S. Pat. No. 3,765,695. Reference is also made to an article "Optical Low Pass Filter For a Single-Vidicon Color Television Camera", by Mino, and Okano, "Journal of the S.M.P.T.E.", Volume 81, page 282 (1972) for background theory.
The U.S. Pat. No. 3,911,479 is cited of general interest to disclose another form of color selective low pass filter.
The desire to provide an economical two dimensional optical low pass filter that is versatile enough to be utilized not only in a one dimensional function, such as in the line scanning of a vidicon, but also, in a two dimensional capacity is still a goal of the prior art.